Yini Xie

Pine wilt disease has caused great economic loss and become an ecological threat since it was introduced into East Asia. In China, Pinus massoniana Lamb. is highly susceptible. The pathogenic of this disease is linked to the invasion of P. massoniana by the pine wood nematode (PWN, Bursaphelenchus xylophilus), leading to various physiological activities. However, the migration pathway of PWN and the defense mechanisms of P. massoniana tissue structure following invasion remain unclear. This knowledge is vital for understanding the pathogenesis of pine wood nematode disease. To address this issue, we analyzed the tissue structure damage, horizontal and vertical migration pathways, and histochemical reactions of P. massoniana after PWN inoculation. The results are as follows: susceptible P. massoniana exhibited more tissue structure damage compared to highly resistant P. massoniana. PWN reproduced and migrated by feeding on and damaging cells. In susceptible P. massoniana, PWN propagated and migrated throughout the entire plant. Highly resistant P. massoniana displayed limited horizontal and vertical migration of PWN, making it challenging for PWNs to move from cambium to xylem. After P. massoniana was damaged by PWNs, a protein cross-linking phenomenon appeared rapidly, with highly resistant P. massoniana exhibiting less protein cross-linking than the susceptible variety. Lignin synthesis is a crucial factor in the tissue defense of P. massoniana. Protein crosslinking provides time for lignin synthesis and is an vital component of tissue defense.

Pinus massoniana is a pioneer species for afforestation timber and oleoresin, while epidemics of pinewood nematode (PWN; Bursaphelenchus xylophilus) are causing a serious biotic disaster for P. massoniana in China. Importantly, resistant P. massoniana could leak copious oleoresin terpenoids to build particular defense fronts for survival when attacked by PWN. However, the defense mechanisms regulating this process remain unknown. Here, PmCYP720B11v2, a cytochrome P450 monooxygenase gene, was first identified and functionally characterized from resistant P. massoniana following PWN inoculation. The tissue-specific expression pattern and localization of PmCYP720B11v2 at the transcript and protein levels in resistant P. massoniana indicated that its upregulation in the stem supported its involvement in the metabolic processes of diterpene biosynthesis as a positive part of the defense against PWN attack. Furthermore, overexpression of PmCYP720B11v2 may enhance the growth and development of plants. In addition, PmCYP720B11v2 activated the metabolic flux of antioxidases and stress-responsive proteins under drought conditions and improved drought stress tolerance. Our results provide new insights into the favorable role of PmCYP720B11v2 in diterpene defense mechanisms in response to PWN attack in resistant P. massoniana and provide a novel metabolic engineering scenario to reform the stress tolerance potential of tobacco.